US9872121B1 - Method and system of processing 5.1-channel signals for stereo replay using binaural corner impulse response - Google Patents
Method and system of processing 5.1-channel signals for stereo replay using binaural corner impulse response Download PDFInfo
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- US9872121B1 US9872121B1 US15/591,283 US201715591283A US9872121B1 US 9872121 B1 US9872121 B1 US 9872121B1 US 201715591283 A US201715591283 A US 201715591283A US 9872121 B1 US9872121 B1 US 9872121B1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S7/00—Indicating arrangements; Control arrangements, e.g. balance control
- H04S7/30—Control circuits for electronic adaptation of the sound field
- H04S7/302—Electronic adaptation of stereophonic sound system to listener position or orientation
- H04S7/303—Tracking of listener position or orientation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S1/00—Two-channel systems
- H04S1/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R3/00—Circuits for transducers, loudspeakers or microphones
- H04R3/12—Circuits for transducers, loudspeakers or microphones for distributing signals to two or more loudspeakers
- H04R3/14—Cross-over networks
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S1/00—Two-channel systems
- H04S1/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
- H04S1/005—For headphones
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S3/00—Systems employing more than two channels, e.g. quadraphonic
- H04S3/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S3/00—Systems employing more than two channels, e.g. quadraphonic
- H04S3/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
- H04S3/004—For headphones
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S3/00—Systems employing more than two channels, e.g. quadraphonic
- H04S3/008—Systems employing more than two channels, e.g. quadraphonic in which the audio signals are in digital form, i.e. employing more than two discrete digital channels
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S3/00—Systems employing more than two channels, e.g. quadraphonic
- H04S3/02—Systems employing more than two channels, e.g. quadraphonic of the matrix type, i.e. in which input signals are combined algebraically, e.g. after having been phase shifted with respect to each other
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S5/00—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S5/00—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation
- H04S5/005—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation of the pseudo five- or more-channel type, e.g. virtual surround
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S7/00—Indicating arrangements; Control arrangements, e.g. balance control
- H04S7/30—Control circuits for electronic adaptation of the sound field
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S2400/00—Details of stereophonic systems covered by H04S but not provided for in its groups
- H04S2400/01—Multi-channel, i.e. more than two input channels, sound reproduction with two speakers wherein the multi-channel information is substantially preserved
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S2400/00—Details of stereophonic systems covered by H04S but not provided for in its groups
- H04S2400/03—Aspects of down-mixing multi-channel audio to configurations with lower numbers of playback channels, e.g. 7.1 -> 5.1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S2420/00—Techniques used stereophonic systems covered by H04S but not provided for in its groups
- H04S2420/01—Enhancing the perception of the sound image or of the spatial distribution using head related transfer functions [HRTF's] or equivalents thereof, e.g. interaural time difference [ITD] or interaural level difference [ILD]
Definitions
- the present application relates to stereo or 2-channel audio processing; and more particularly, to a method for mixing 5.1 channel audio signals into stereo or 2-channels speaker playback surround sound signals using binaural corner impulse responses (BCIR) as filter in order to obtain better surround sound and audio quality.
- BCIR binaural corner impulse responses
- Binaural audios consist of reproducing at the entrance of each the listener's ear canals the sound pressure signals containing the proper interaural time difference (ITD) and interaural level difference (ILD) cues required for the listener to perceive a realistic 3D sound image or sound-field.
- binaural audio relies on recording sound with microphones implanted in the ear canals of an artificial human head or equivalently, numerically convolving digital audio with a head-related transfer function (HRTF) representing the listener's head, then playing back the recorded stereo signals at or near the listener's ear canal entrances through earphones or headphones.
- HRTF filtered digital sounds provide interaural time difference (ITD) and interaural level difference (ILD) cues to listeners' left and right ears, allowing listeners to perceive sounds with distance and spatial feelings without being in such an environment.
- the playback speakers include a front left (FL), a central (C), a front right (FR), a left surround (LS), a right surround (RS) and a subwoofer (LFE), the configuration and positioning of the speakers are, however, complicated and very expensive.
- Much effort have been made to simplify the multi-channel playback systems by down-mixing 5.1 channel audio signals into two-channel sounds so that listeners with two speaker systems or headphones can receive similar spatial and dimensional effects as those of multichannel systems.
- Down-mixing is the audio process of converting audio signals from multiple-channel input into an output of audio signals using fewer channels. Audio mixing of 5.1 channel audio is a complicated task that utilizes multiple functions in order to create a distinct and clear stereo sound.
- surround sound channels LS and RS
- the center channel C
- the LFE channel is either mixed with the front channels or removed completely, during which digital faders are used to attenuate or boost the audio levels of one or several particular channels
- equalizers alter the frequency response of the audio sound to affect the tones of the different frequencies.
- Down-mixing is conducted in many of today's electronics, such as DVD players or headsets. Programs as MPEGs and DOLBY Digital decoders may be used to conduct the proper automatic filtering and equalization in order to produce a stereo sound from multiple channels with minimal distortions.
- HRTFs filters have been built to render sense of space and dimensional image to listeners. HRTFs are obtained through measuring the impulse responses at the left ear and the right ear. Conventionally in order to simplify HRTFs computation, measurements are usually undertaken in an anechoic chamber or in a reflective room to avoid the influence of the environment. However, HRTFs measured in an anechoic chamber do not completely reflect a real hearing experience such as at a concert or in a normal room condition. Audio signals processed with such HRTF filters do not provide the same distance and spatial feelings as when listeners hear in a home or in a normal and non-anechoic chamber surrounding.
- BRTFs binaural room transfer functions
- BRIRs binaural room impulse responses
- the reverberations of the sounds are produced when a sound is reflected off of a surface, such as a wall, furniture, or even air.
- Reverberation improves realism.
- the amount of reverberation can be used for the construction of the effects of environment of concert halls so that they produce the best acoustics within the occupied space.
- the impulse responses collected in a normal room are thus processed to simulate the reverberation of sound within the room location.
- HRTFs head-related transfer functions
- DSP digital signal processor
- the present application discloses a method of down-mixing audio signals of 5.1 channels into stereo channels with an enhancement in subwoofer signals and binaural processing using DSP chips.
- input audio signals from front left (FL), a central (C), a front right (FR), a left surround (LS), a right surround (RS) and a subwoofer channels are mixed for binaural playback wherein power of the subwoofer signals are not reduced but are downward dynamic compression (DRC) processed before mixing for double channel stereo play back.
- FL front left
- C central
- FR front right
- LS left surround
- RS right surround
- DRC downward dynamic compression
- input audio signals from front left (FL), a central (C), a front right (FR), a left surround (LS), a right surround (RS) and a subwoofer channels are mixed for binaural playback wherein the powers of the left surround (LS) and the right surround (RS) are first enhanced and then the enhanced signals are downward dynamic compression (DRC) processed before mixing for double channel stereo playback.
- FL front left
- C central
- FR front right
- LS left surround
- RS right surround
- subwoofer channels subwoofer channels
- binaural audio signals are processed using binaural room transfer functions (BRTFs) as a filter wherein binaural corner impulse responses (BCIR) is obtained to build the filter in order to produce sounds having better realism and spatial distance perception.
- BRTFs binaural room transfer functions
- BCIR binaural corner impulse responses
- the binaural corner impulse responses are measured in a room of the size 5 m ⁇ 3 m ⁇ 3 m and by placing an artificial human head with microphones implanted in the ear canals at one corner of the room and placing a loudspeaker at the diagonal corner of the room where the two corners are diagonal to each other, and the left rear or the right rear side of the head faces to the speaker.
- binaural room transfer functions are obtained by cutting off the initial 14.1 mini-seconds of the response time and by collecting the first 1024 sample taps after the 14.1 mini-seconds of cutting off time-window and by Fourier transform processing of the impulse response taps.
- the filtering processing is performed by using ADAU1701 DSP chip of Analog Devices, Inc (ADI).
- FIG. 1 illustrates the most commonly used process of down-mixing 5.1 channel audio signals into binaural audio signals for two channel playback.
- FIG. 2 depicts the general method of binaural audio signal processing in traditional down-mixing 5.1 channel audio signals shown in FIG. 1 .
- FIG. 3 illustrates an example improved process of down mixing 5.1 channel audio signals into binaural audio signals for two channel play backs in accordance with this application.
- FIG. 4 depicts an example method of binaural audio signal processing in an example down-mixing 5.1 channel audio signals shown in FIG. 3 in accordance with this application.
- FIG. 5 figuratively depicts the measurement room setting for measuring BCIR data in accordance with this application.
- FIG. 6 shows example graphs of measured impulse responses in accordance with this application.
- FIG. 7 shows the data graphs of the measured impulse responses of FIG. 6 with the initial cut off time window in accordance with this application.
- FIG. 8 shows example graphs of collecting 1020 impulse response data points of FIG. 7 in accordance with this application.
- FIG. 9 shows an example contrast of the output audio signals at the left ear channel and the output audio signals at the right ear channel after applying the BRTF filter built thereof in accordance with this application.
- 5.1 channel audio signals refer to audio signals for playbacks from different directions including audio signals for playbacks at a front left (FL), a central (C), a front right (FR), a left surround (LS), a right surround (RS) and a subwoofer channels.
- down-mixing refers to the layback process of audio sound signals for multiple channels, such as 5.1 channel audio, into a playback system of less channels.
- impulse response refers to the measurement in which there is a reaction from a person or system in response to an external sound source. They give the acoustic characteristics of a location. The measurements collected can be processed in order to simulate the reverberation of the sound within the location.
- reverberation refers to the persistence of sound after it is produced. A reverb is produced when a sound is reflected off of a surface, such as a wall, furniture, or even air. The amount of reverberation can be used for the construction of concert halls in order to produce the best acoustics within the occupied space.
- HRTF head related transfer function
- binaural rendering refers to the process that makes use of the knowledge of transfer functions between sound sources and the listener's ear signals to create virtual sound sources which are placed around the listener. This process involves convolving a monophonic sound signal with a pair of HTRFs or BRTFs to produce ear signals so that the output audio signals can be played at the ear as if they are played in a real room.
- HTRFs or BRTFs the important cues for spatial hearing are conveyed and users are able to localize sounds in direction and distance and to perceive envelopment, sounds appear to originate somewhere outside the listener's head as opposed to the in-head localization with a conventional stereo headphone reproduction.
- the quality of binaural rendering is mostly determined by the localization performance, front-back discrimination, externalization and perceived sound coloration.
- binaural replay signals of 5.1-channel refers to a method of signal processing of audio signals for the 5.1 channel audio playback systems to be played with binaural effects of space and distance perception to human listeners.
- binaural corner impulse response refers to a method of measurement of impulse response using a room (width 3 meter, length 5 meter, height 3 meter) with a reverberation time of about 450 mini-seconds and placing an artificial human head and a loudspeaker at the diagonal corners of the room.
- This 3D surround effect is typically provided by multi-channel replay system placed around the listener.
- the most widely used multi-channel replay system is the 5.1 home theatre systems.
- This system comprises of a bass channel (LFE) and 5 full-band channels wherein left (FL), middle (C), and right (FR) channels are in the front and left surround (LS) and right surround (RS) channels are in the back.
- Horizontal surround effects can be reproduced by 5.1-channel signals through a properly configured 5.1 system replay.
- the problem with such multi-channel replay systems are the higher prices and more complex installation. Therefore, people would like to choose a more simple system to achieve the 3D surround effect that they desire.
- C channel 105 and LFE channel 107 are respectively attenuated by ⁇ 3 dB (process 113 and 115 ) and each is then fed to both the left channel 119 and right channel 121 of a dual channel system.
- FL channel 101 is fed to the left channel 119 and FR channel 103 is fed to the right channel 121 without change, while LS channel 109 and RS channel 111 signals are filtered with HRTF filtering process 117 before being fed to the left channel 119 and right channel 121 respectively for down-mixing.
- the left output signal L′ 123 and the right output signal R′ 125 are binaural audio signals that render the effects of 5.1 audio channels with space and distance perception.
- FIG. 2 figuratively illustrates the conventional binaural rendering process 117 of HRTF filtering.
- LS channel signals 201 and right channel signals 203 are first respectively treated as a virtual sound source from a defined location away from head 237 .
- LS channel signals 201 are then filtered with a pair of HRTFs H C 205 and H I 207 and RS channel signals 203 are also filtered with a pair of HRTFs H C 211 and H I 209 .
- the output of H C filtered LS signal 201 and the output of H I filtered RS channel signals 203 are then summed up to form left channel E L 213 .
- the output of H I filtered LS signal 201 and the output of H C filtered RS channel signal 203 are then summed up to form left channel E R 215 .
- E L 213 and E R 215 signals are further processed for signal crosstalk cancellation with matrix elements A 11 , A 21 , A 12 , A 22 at step 217 , 219 , 221 and 223 .
- the output signals (L′ and R′) are summed up at steps 225 and 227 to be the source audios for left and right ear respectively.
- the related HRTFs from L′ and R′ to head 237 are H LL , H RL , H LR , H RR .
- the replay effect of the processed loudspeaker system L′ and R′ is equivalent to that of the headphones, H LL , H RL , H LR , H RR represent sound source L′ and R′ transmitting from their virtual positions to the left and right ear with angles of ⁇ and ⁇ .
- the HRTF data used above are usually derived from experimental measurements, which often include the following two processes: Measurements are made in an anechoic chamber with no reflected sound; measurements are made by an artificial head instead of real people.
- FIG. 3 To address the above described problems in down-mixing multichannel audio signals for binaural replays, an improved down-mixing process and virtual signal processing is described in FIG. 3 to improve bass and surround sound effect as well as the spatial and distance perception.
- bass channel LFE 307 signals are treated with ⁇ 3 dB dynamic range compression (DRC) processing 315 and 321 before being fed to the left 327 and right channel 331 for down-mixing to improve bass effect.
- DRC dynamic range compression
- the traditional ⁇ 3 dB magnitude attenuation is omitted to enhance the amplitude of the bass.
- the ⁇ 3 dB DRC unit avoids distortion of the bass signals or the interferences of other channels due to excessive bass signal amplitude.
- BCIR data is used for BRTF filters 317 and 319 to process LS 309 and RS 311 to generate binaural signals of virtual source from the left and right rears for replay.
- LS channel 309 and RS channel 311 signals are boosted by +3 dB at step 323 , and before being fed to the left 327 and right channel 331 , they are again processed through a ⁇ 3 dB DRC unit to reduce the impact of being masked or disturbed by other channels.
- G C and G I represent elements after combining crosstalk cancellation and binaural effect filtering. Respectively treated as a virtual sound source from a defined location, LS channel signals 401 are filtered with a pair of BRTFs G C 405 and G I 417 and RS channel signals 403 are filtered with a pair of BRTFs G C 409 and G I 407 . The output of G C filtered LS signal 401 and the output of G I filtered RS channel signals 403 are summed up to form left sound source L′.
- G C ⁇ ⁇ ⁇ CH C - ⁇ ⁇ ⁇ CH I ⁇ 2 - ⁇ 2 ( 8 )
- G I ⁇ ⁇ ⁇ CH I - ⁇ ⁇ ⁇ CH C ⁇ 2 - ⁇ 2 ( 9 )
- L′ and R′ represent the left and right channel signals that are ultimately fed to the targeted dual-channel sound system.
- the operator ⁇ represents the DRC process of ⁇ 3 dB.
- G C and G I represent the process of combining crosstalk cancellation and generating binaural signals of a virtual sound source.
- 0.707 represents the attenuation of ⁇ 3 dB and 1.414 represents enhancement of +3 dB in signal magnitude.
- BCIR data is obtained by using a high-quality HIFI speaker 509 and artificial head 503 around 4.8 meters in distance 505 from each other to measure impulse responses within a 5 meters length 501 by 3 meters width 507 room and by 3 meter height with a reverberation time of about 450 mini-seconds.
- the loudspeaker and the microphone are placed at the opposite corners of the room in order to obtain a better sense of spatial hearing and make subsequent signal processing easier. The good sound quality of this room was verified by about several audio sound experts in test hearing.
- the HRTF data measured in the anechoic chamber using the artificial head can distort the direction of the resulting virtual sound source and seriously affect the surround sound 3D effect.
- Many studies have pointed out that the reflected sound in the environment plays a very important role in the localization of the sound source. In fact, a regular listener with normal hearing can clearly perceive this.
- the sensed sound source position in the anechoic chamber is significantly closer to the listener than the position in the normal room. That is to say, the absence of reflected sound compromises the perception of spatial location. Therefore, this invention uses the BRIR data, including the reflected sound, to perform the data processing when the binaural signals of virtual sound source are generated.
- BRIR data which includes reflected sounds
- the presence of reflected sounds greatly increases the length of the impulse response, especially when measuring impulse response data in a room with long reverberation time, making it very difficult to design the filter.
- the number of generated filter taps is too large for the common DSP to be able to handle.
- HRTF filters the energy of the HRTF data measured in the anechoic chamber is almost concentrated in the first 128 data points. Because the actual measured data is firstly a binaural impulse response that does not contain reflected sounds, the process usually lasts about only 2.67 mini-seconds at a sampling frequency of 48 kHz and it only requires 128 taps to design a filter of very acceptably small error. But for the typical home environment, the reverberation time can be as long as hundreds of milliseconds, common DSP chips are incapable of handling so many data points.
- the first approach taken in this invention selects the measurement environment in a relatively small room having a width of about 3 meter, a length of about 5 meter, a height of about 3 meter, and a reverberation time of about 450 mini-seconds.
- a number of experienced audio practitioners' auditions showed that listeners were able to hear a good sense of space with the loudspeaker in the room playing a variety of audio sources.
- FIG. 6 an example left ear 601 and right ear 603 impulse response are shown.
- the time for the direct sound to reach the artificial head from the loudspeaker is about 14.1 mini-seconds 605 before there is impulse response time 607 .
- the silent time 605 is then removed ( FIG. 7 ) to generate left ear graph 701 and right ear graph 703 .
- Numeral reference 801 represents measured impulse data from left ear and numeral reference 803 represents measured impulse data from right ear.
- the Fourier transform can be carried out to obtain the corresponding transfer function CH C and CH I in equation (4) and then obtain G C and G I according to equations (8) and (9).
- the calculated G C and G I may need cycle delay processing so that it represents the causal system, and appropriate smoothing to reduce the length of the impulse response data obtained through inverse Fourier transform.
- a BRTFs filter of IIR (infinite impulse response) digital filter structure may be constructed with Prony method (a well known method in the field) at a power (128,128).
- Prony method a well known method in the field
- the entire signal process was carried out using ADAU1701 DSP chip of the ADI Corporation.
- example filtered binaural audio signals are generated for the left ear ( 901 ) and for the right ear ( 903 ) using a BRTFs filter of IIR filter as described above constructed with ADAU1701 DSP chips.
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Abstract
Description
L′=L+0.707C+∇(LFE)+∇[1.414(G C LS+G I RS)] (10)
R′=R+0.707C+∇(LFE)+∇[1.1414(G I LS+G C RS)] (11)
Claims (24)
L′=L+0.707C+∇(LFE)+∇[1.414(G C LS+G I RS)]
R′=R+0.707C+∇(LFE)+∇[1.1414(G I LS+G C RS)]
L′=L+0.707C+∇(LFE)+∇[1.414(G C LS+G I RS)]
R′=R+0.707C+∇(LFE)+∇[1.1414(G I LS+G C RS)]
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US20060018493A1 (en) * | 2004-07-24 | 2006-01-26 | Yoon-Hark Oh | Apparatus and method of automatically compensating an audio volume in response to channel change |
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US11523239B2 (en) * | 2019-07-22 | 2022-12-06 | Hisense Visual Technology Co., Ltd. | Display apparatus and method for processing audio |
CN110933588A (en) * | 2019-12-02 | 2020-03-27 | 安徽井利电子有限公司 | Method for simplifying five-loudspeaker system into four-loudspeaker system |
FR3140727A1 (en) * | 2022-10-11 | 2024-04-12 | Franck Rosset | Method of reproducing an encoded audio signal |
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